Lithography is a process in which printing takes place from a plate with specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, are incapable of accepting the ink. The areas of the medium being printed which accept ink from those plate areas which have accepted ink, form the printing image areas, while those in contact with those plate areas unable to accept ink, form the background areas. For the purposes of the present invention areas of the lithographic plate capable of accepting the ink will be referred to as hydrophobic and those incapable, after prewetting with water, of accepting this ink will be referred to as hydrophilic.
In the art of negative-working lithography, printing plates are produced in which image areas of the plates are incapable, after prewetting with water, of accepting the oily and greasy lithographic ink (hydrophilic) and non-image areas are capable of accepting this ink (hydrophobic). Two variants are possible depending on whether the surface of the plate before image-wise treatment is hydrophobic or hydrophilic. In the variant in which the surface of the plate before image-wise treatment is hydrophobic, image-wise treatment renders the image areas hydrophilic, whereas in the variant in which the surface of the plate before imaging is hydrophilic, image-wise treatment corresponding to the negative of the image renders the non-image areas hydrophobic.
In the art of positive-working lithography, printing plates are produced in which image areas are capable of accepting the oily and greasy lithographic ink (hydrophobic) and non-image areas are incapable, after prewetting with water, of accepting this ink (hydrophilic). Two variants are also possible depending on whether the surface of the plate before image-wise treatment is hydrophobic or hydrophilic. In the variant in which the surface of the plate prior to image-wise treatment is hydrophobic, image-wise treatment corresponding to the negative of the image renders the non-image areas hydrophilic, whereas in the variant in which the surface of the plate before image-wise treatment is hydrophilic, the image-wise treatment renders the image-areas hydrophobic.
Image-wise treatment, or image-wise treatment corresponding to the negative of the image, for the production of lithographic printing plates can be carried out using actinic radiation, ionized radiation or heat, or a combination thereof in combination with appropriate processing of the plates.
The use of thermographic or photothermographic materials in the production of lithographic printing plates opens the possibility of using dry development techniques, which have the advantage, over conventional processes requiring development with externally applied (wet) developers, of being entirely dry processes developable without additional ingredients.
Thermal imaging or thermographic processes utilizing the reduction of substantially light-insensitive organic heavy metal salts by organic reducing agents to heavy metal particles upon the application of heat, are entirely dry processes developable without additional ingredients. Furthermore, such systems can be rendered photosensitive by the incorporation of a photosensitive agent capable after exposure of catalyzing the thermal reduction of the organic heavy metal salts by the reducing agents.
A process for forming an image is described in U.S. Pat. No. 4,210,711, comprising the image-wise exposure of a photosensitive image-forming material composed of a support and a layer of a thermoplastic photosensitive composition which is not adhesive at ordinary temperatures, heating the exposed photo-sensitive material in intimate contact with a peeling development carrier sheet at least one surface of which is composed of a thermoplastic material not adhesive at ordinary temperature to a temperature above the softening temperature of at least one of the photo-sensitive composition layer and the peeling development carrier-sheet from the photo-sensitive image-forming material at about or below the heating temperature thereby to form the exposed or unexposed area of the photo-sensitive composition layer on the peeling development sheet, and the corresponding unexposed or exposed area on the support, respectively as a separate image wherein the photo-sensitive composition contains a photosensitive polyhalogen compound, a photosensitive quinone, a photosensitive polymer, a silver halide or an organic silver salt.
A thermographic lithographic printing plate is described in U.S. Pat. No. 3,685,993 with a composition comprising a complex of a heat fusible phenolic resin and a high molecular weight water-soluble poly-ether, -amine or -amide polymer, and an organic silver salt oxidizing agent for the resin, the composition undergoing a visible change upon heating to form ink-receptive image areas.
The difference in ink-receptive behaviour of image and non-image areas in prior art thermographic processes for the production of lithographic plates is insufficient to yield high quality prints. Furthermore, such lithographic plates have sub-optimal printing properties e.g. insufficient ink-acceptance in the hydrophobic areas of the lithographic plate, low printing endurance etc.